They said the victims included the local leader of the "Sahwa" (Awakening) tribal militia, along with two of his sons.

Sahwa fighters, who helped U.S. troops turn the tide against al Qaeda-linked militants in Iraq from 2006 onwards, are often targeted by insurgents battling the Shi'ite-led government.

Two years after U.S. troops left Iraq, violence has climbed back to its highest levels since the Sunni-Shi'ite bloodletting of 2006-2007, when tens of thousands of people were killed.

The United Nations says nearly 9,000 people died violently in Iraq last year, all but 1,050 of them civilians.

Tension has been high this year since al Qaeda-linked militants and other Sunni insurgents seized the city of Falluja on Jan. 1, exploiting grievances among minority Sunnis to regain strength in the western desert province of Anbar.

The toll from violence on Saturday rose to at least 32 killed, including six men involved in an attack on a juvenile prison in Baghdad, and 75 wounded, police and medical sources said.

In a late-night attack, gunmen killed two off-duty soldiers travelling in a car near the refinery town of Baiji, 180 km (110 miles) north of Baghdad, police said. (Reporting by Ahmed Rasheed; Writing by Alistair Lyon; Editing by Pravin Char)

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